Electrical impedance tomography (EIT) devices are known from the state of the art. These devices are configured and intended to generate signals obtained by electrical impedance measurements and data and data streams obtained therefrom, an image, a plurality of images or a continuous series of images. These images or series of images show differences in the conductivity of various body tissues, bones, skin, body fluids and organs, especially of the lung, which are useful for observation of the patient situation.
Thus, U.S. Pat. No. 6,236,886 describes an electrical impedance tomograph comprising an array of a plurality of electrodes, current feed to at least two electrodes, signal acquisition at the other electrodes and a method with an algorithm for image reconstruction to determine the distribution of conductivities of a body, such as bones, skin and blood vessels in a basic configuration comprising components for signal acquisition (electrodes), signal processing (amplifier, A/D converter), current feed (generator, voltage-current converter, current limitation) and control components.
It is stated in U.S. Pat. No. 5,807,251 that it is known to provide a set of electrodes, which are arranged at a defined distance from one another, for example, around the thorax of a patient in electrical contact with the skin, in the clinical application of EIT. Electrodes arranged adjacent to one another, each alternating between different electrodes or all of the possible pairs of electrodes, are applied for an electrical current or voltage input signal. While the input signal is applied to one of the pair of electrodes arranged adjacent to one another, the currents or voltages between each pair of remaining electrodes adjacent to one another are measured, and the measured data are processed in a known manner in order to obtain a visualization of the distribution of the specific electrical resistance over a cross section of the patient, around whom the ring of electrodes is arranged, and to display same on a display screen.
Unlike other imaging radiological methods (X-ray machines, radiological computer tomographs), electrical impedance tomography (EIT) has the advantage that radiation exposure, which is disadvantageous for the patient, does not occur. Unlike sonographic methods, a continuous acquisition of images over a representative cross section of the entire thorax and the lungs of the patient by means of the electrode belt can be carried out using EIT. In addition, the need to use a contact gel, which has to be applied before each examination, is omitted. Electrical impedance tomography (EIT) thus offers the advantage of making a continuous monitoring of the lungs possible in order to observe and to document a course of therapy of a mechanically ventilated or spontaneously breathing patient.
As is known, for example, from U.S. Pat. No. 5,807,251, an impedance measurement is carried out using an EIT device on the thorax by means of an electrode array around the thorax of a patient, and an image of the lungs of the patient is generated from the impedances by means of a conversion to the geometry of the thorax.
With a total number of, for example, 16 electrodes arranged around the thorax of a patient, an EIT device can generate a 32×32-pixel image of the lungs in a circulation of current feeds to each of two electrodes and a pickup of voltage-measured values (EIT measured signals) at the remaining electrodes. In this connection, at the 16 electrodes, a number of 208 impedance measured values is acquired at the electrodes. A quantity of 1,024 pixels is then obtained from these 208 impedance measured values using the EIT image reconstruction.
The electrodes are arranged in a horizontal array around the thorax of a living being and comprising an area of the lungs of the living being for carrying out the electrical impedance tomography (EIT). This results in a position in the plane of the electrode array, which can be designated as a thoracic-axial position of the electrode array on the circumference of the transverse plane of the body.
When an electrode belt is used as an electrode array, in or at which the electrodes are arranged and held at fixed positions with defined distance to one another, the chances of deviations in the vertical position between adjacent electrodes to one another on the thorax are comparatively small. Thus, a vertical shift during the image reconstruction, in which a horizontal tomogram through the thorax is determined as a so-called dorsal view, plays a comparatively minor role in the positioning of the electrode belt on the thorax. The horizontal tomogram is in this case imaged inclined only by a few degrees. In addition, electrical fields are obtained not only in the section plane itself, but also in areas above and below of approx. 5 cm to 10 cm of the section plane each, which are then also incorporated into the impedance measurements in any case, during the electrical feed to the electrodes. As a consequence, the only slightly possible inclination of the electrode belt in the tidal image is virtually imperceptible as an effect. Moreover, a comparatively reproducible and representative horizontal position of the electrode belt during application can be achieved due to an orientation at the costal arches, so that possible errors in the horizontal attachment of the electrode belt to the thorax occur rather rarely.
By contrast, the horizontal position of the electrodes is important because the physiologically expected, almost elliptical geometry of the thorax, which deviates from the ideally round, circular or cylindrical shape, is important for the generation of the dorsal view because information is necessary for an incorporation of the elliptical shape into the image reconstruction on which electrode is arranged on the thorax and at which position, i.e., front (sternum), lateral (costal arches) or rear (spine). Various types of living beings, in whom it is common for gas exchange to occur in them by means of lung breathing, have each circumferential shapes, which are typical in themselves, in the body structure surrounding the lungs (muscles, skeleton, organs, body tissues, skin). A deviation of the shape of the thorax from an ideal circular shape is, for example, given in human beings, in principle, to the effect that the typical circumferential shape is, as a rule, a rather elliptical than circular shape. In other living beings (patients of other species), such as horses, dogs, pigs, rodents or birds, other typical shapes of the circumferential shapes result depending on the species of animal. In this respect, an idea of the present invention with the electrical impedance tomography device with determination of a position of an electrode array associated with the electrical impedance tomography device can be extrapolated not only to the application of electrical impedance tomography in human beings, but also to a broad biodiversity of the animal kingdom. In human beings, especially the area of the electrode plane being used during the electrical impedance tomography (EIT), i.e., the plane in the horizontal section through the thorax, approximately in the area of the third, fourth, fifth costal arch, as well as of the fifth, sixth, seventh thoracic vertebra, has an elliptical circumferential shape (ellipsoid) in the area of the upper part of the body and of the thorax. In addition to the geometric shape with an anatomically predefined elliptical configuration area, there are additional characteristic features, which have an effect on the impedance, impedance differences and impedance distribution measured by means of EIT and have an effect during the image reconstruction on the visualization of the ventilation of the lungs in a transverse view, or are taken into consideration in the mathematically-algorithmically applied manner of the image reconstruction. Thus, in addition to the lung, with areas supplied with blood in the rhythm of the heartbeat and ventilated in the rhythm of the breathing cycle, the heart, with areas supplied with blood essentially in the rhythm of the heartbeat, but quasi permanently in a similar manner, elements of the skeleton, the sternum in the front-side area of the upper part of the body and the spine in the rear-side area, the impedances of which are both unaffected by blood supply and breathing cycle, are, in addition, arranged in this electrode plane. Both the sternum and especially the spine have instead an impedance that deviates from these areas and other areas (heart, skin lungs, tissue) in the electrode plane and is essentially constant.
An ellipse can be described as a closed oval shape, as a deviation from a circular shape, and as a dimensionless number by means of so-called eccentricity.
In this case, eccentricity describes the relationship of the two half-axes, which are at right angles to one another. Both half-axes are of identical length in case of a circle, and an ellipse is defined by a shorter half-axis and a longer half-axis, compared to the shorter half-axis, of different lengths. Due to the elliptical shape and depending on the eccentricity thereof, different distances arise between the feeding electrodes and the measuring electrodes located opposite them depending on whether the feed takes place on the front-side area with measurement on the rear-side area, or the feed takes place on the rear-side area with measurement on the front-side area along the frontal plane of the human body, or whether the feed takes place on the left side of the body with measurement on the right side of the body, or the feed takes place on the right side of the body with measurement on the left side of the body along the transverse axis of the human body. The two feed/measurement constellations located opposite one another on the left/right side of the body represent feeds/measurements on the longer half-axis in the elliptical circumferential shape of the human body and the two feed/measurement constellations located opposite one another on the front side of the body/on the rear side of the body represent feeds/measurements on the shorter half-axis in the elliptical circumferential shape of the human body. The shape and the differences in the distances between the feeding electrodes and the measuring electrodes located opposite them, which differences result therefrom, have no effect only in case of a circular circumference, as, for example, in a pig. In this case, however, at least the elements of the skeleton or of the organs still bring about differences in the impedances between the feeding electrodes and the measuring electrodes located opposite them.
A practical example shall illustrate the effect of how an axial torsion of an electrode belt arranged in a horizontal position has an effect.
A circumferential shift—lateral or axial twist angle of the electrode belt comprising a number of 16 electrodes in a horizontal alignment around the thorax by the distance of one electrode results in a difference from the preset and expected alignment of about 0.05 m, corresponding to a torsion angle of 22.5°, in case of a human patient with an average diameter of the thorax of 0.80 m. The difference is correspondingly reduced to about 0.025 m, corresponding to a torsion angle of 12.25°, in case of a number of 32 electrodes.
An electrical impedance tomography system is shown in WO 2015/048917 A1. The EIT system is suitable for acquiring electrical properties of a lung of a patient as impedances. For this, the impedance values and impedance changes of the lung are acquired by means of feeding voltage or current between two or more electrodes and a signal acquisition at an electrode array and then further processed by means of data processing. The data processing comprises a reconstruction algorithm with a data processor in order to determine and to reconstruct the electrical properties from the impedances. In case of the reconstruction of the electrical properties from the acquired measured data, an anatomical model is selected from a plurality of anatomical models on the basis of the biometric data of the patient and the reconstruction of the EIT image data is adapted on the basis of the anatomical model and of the biometric data. This adaptation requires that biometric data be inputted into the system by the user. In this connection, age, gender, height as well as a circumference of the thorax of the patient are to be inputted under biometric data. This means that boundary conditions are to be inputted by the user before the beginning of the measurement with the electrical impedance tomography system in order to be able to select and apply a suitable reconstruction model, which is based on a selected anatomical model.
For the operation of electrical impedance tomography systems, it is not really advantageous in many application situations to have to measure or acquire and input a plurality of patient data before the beginning of the application. In particular, the need for data that refer to the properties of the body such as circumference of the thorax, height and weight assumes that the anatomical models stored in the device can also be used for a plurality of patients.
In addition to this, the problem of how the electrode belt is placed or positioned on the body is not addressed by WO 2015/048917 A1. It is not possible to determine a horizontal position of the belt, as well as especially not an axial torsion or shifting about the vertical axis of the body (longitudinal or sagittal or front plane of the body), even by inputting biometric data of the patient. Thus, the correct positioning of the electrode belt remains the responsibility of the user in spite of applying anatomical models and taking biometric data into consideration.